The overall objective of this proposal is to understand the molecular mechanism whereby bacteriophage T7 is restricted upon infection of a male strain of E. coli. This phenomenom is also known as the abortive infection. A new approach is being applied to help solve an old and controversial problem. We have made, via an in vivo recombination experiment, hybrid phages between T7 and the related phage T3. Whereas T7 is restricted by males, T3 is not and these hybrid phages, which are predominately T7 in origin, contain the T3 functions necessary to overcome the abortive infection. We have also recently isolated mutants of T3 which appear to be fully restricted by male strains. Proposed here is a more detailed genetic and physiological characterization of the gybrid phages, designed in particular to separate the two essential functions supplied by T3. These derivatives will be characterized independently on infections of both male and female strains. Similarly the existing T3 mutants, together with others that will be isolated, will be characterized by genetic, physical and physiological criteria. Given these data, in particular knowing precisely which gene products are defective in the T3 mutants, or prevented from functioning (in the case of T7) by the presence of F we can then study the abortive infection itself. For the first time this phenomenom can be investigated by the use of phage mutants and thus the extensive body of knowldege accumulated, on T7 gene products in particular, can be applied to an understanding of the cause of the F factor-mediated abortive infection.